1. Field of the Invention
The present invention relates to a process and an apparatus for producing a thin-webbed H-beam steel having a web thinner than the flanges thereof, in which the occurrence of a wavy web is prevented, the hardening of the flange surfaces when forcibly cooled is suppressed, the time of cooling a hot-rolled H-beam is reduced, and a precise control of the cooling is ensured.
The term "thin-webbed H-beam steel" as used herein denotes an H-beam steel having a web thinner than that specified by the "Japanese Industrial Standard (JIS)" or "American Society for Testing and Material (ASTM)", or having a flange/web thickness ratio larger than that specified thereby.
An H-beam steel is also sometimes referred to as "H-section steel".
2. Description of the Related Art
The thin-webbed H-beam steels, which have a large section modulus with respect to a weight per unit length and are very economical, are usually produced as a welded or built-up H-beam, but recently processes for producing H-beams by hot-rolling have been proposed. One of the most important objects thereof is to prevent the occurrence of a wavy web during cooling of a hot-rolled H-beam, and several practical solutions to this problem have been proposed.
It is well known in the art that the waviness of a web of a thin-webbed H-beam steels occurs when a compressive stress in the web exceeds the buckling strength of the web, which is induced by a residual internal stress due to a difference between the temperatures of the web and the flanges of an H-beam during the cooling from a final hot-rolling temperature.
The present inventors and others have proposed a solution to the problem of a wavy web as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 1-205028, which comprises forcibly water cooling the flanges of a final-hot-rolled H-beam steel immediately after the completion of the hot rolling. To ensure that a difference between the flange and the web temperatures upon completion of the cooling is within a desired range, the forcible cooling of the flanges is effected in a manner such that either the cooling time is not longer than an upper limit or the difference between the flange and the web temperatures upon completion of the cooling is not less than a lower limit, as a wavy web does not occur during the cooling within such upper and lower limits, and such that either the cooling time is not less than a lower limit or the difference between the flange and the web temperatures upon completion of the cooling is not more than an upper limit, as a thermal stress, generated in the web during air cooling to room temperature, does not exceed a buckling strength of the web within such lower and upper limits. These upper and lower limits are predetermined with respect to the size of H-beam and the flow quantity density of the coolant water.
The present inventors also proposed another solution to the problem of the occurrence of a wavy web, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 60-248818, in which a lower limit of the web temperature, at or above which the wavy web occurs during the cooling of a hot-rolled H-beam, is predetermined with respect to the size of a hot-rolled H-beam, an H-beam having a temperature higher than the lower limit is gripped at the longitudinal ends to suppress the thermal contraction, the flanges are then forcibly cooled to cause a tensile plastic deformation in the section, and simultaneously, reduce the difference between the flange and the web temperatures, and thereafter, the ends are freed from the suppression.
Japanese Unexamined Patent Publication (Kokai) No. 58-34130 discloses a process for producing an H-beam steel having a reduced residual stress. This process determines a difference between the flange and the web temperatures corresponding to a target residual stress, composes a formula expressing a residual stress of H-beam as a function of the flange and the web temperatures upon completion of the final hot rolling and of the section size of the hot-rolled H-beam, and by using the formula, determines a final hot rolling temperature corresponding to the target residual stress, and adjusts the flange and the web temperatures to the thus determined final hot rolling temperature.
Japanese Unexamined Patent Publication (Kokai) No. 62-174326 discloses an apparatus for spraying a coolant over the flanges of an H-beam steel, using two types of nozzles to spray a coolant over different portions of the flange so that a uniform temperature distribution is ensured for the flange and the flange cooling efficiency is also improved. More specifically, the two types of nozzle include a nozzle for spraying a coolant generally over the whole flange surface and a nozzle for spraying a coolant selectively over mid-width portion of a flange, which has a highest temperature.
Japanese Examined Patent Publication (Kokoku) No. 57-59003 discloses a process in which the flange and the web temperatures of an H-beam, which has not been final-hot-rolled, are measured, the temperatures of predetermined cooling spots of the H-beam are estimated from the measured temperatures, a difference between the estimated temperatures is determined, and the cooling power of forcible cooling apparatuses provided at the predetermined cooling spots is adjusted based on a gap between the estimated temperature difference and an allowable temperature difference. The cooling power is usually expressed as a product of the following three terms: a flow quantity per unit area or a flow quantity density; the flange width; and a flange length over which the cooling is effective or a cooled flange length. A known apparatus used for the forcible cooling comprises a coolant water path having a flow regulating valve and a flow meter inserted therein in series, in which the coolant flow is adjusted by a feedback control.
The above-mentioned Japanese Patent Publication Nos. 60-248818 and 1-205028 use a forcible cooling effected in a manner such that the difference between the flange and the web temperatures is reduced, to provide a basic solution to the problem of the occurrence of a wavy web of thin-webbed H-beam steels during cooling. The former, Japanese Patent Publication No. 60-248818, however, has a disadvantage in that it needs a lot of equipment for suppressing the thermal contraction of an H-beam steel; the latter, Japanese Patent Publication No. 1-205028 does not need such special equipment and is more advantageous in practical use.
Nevertheless, in Japanese Patent Publication No. 1-205028, the allowable cooling time range between the upper and the lower limits must be very small in accordance with the flange and the web temperatures upon the initiation of the flange cooling, and particularly, when the web has a very small thickness or the flange/web thickness ratio is very large, a variation of temperature over the length of a rolled material causes a problem such that the wavy web cannot be completely prevented, and in turn, the product yield is lowered. It is also disadvantageous in that, to prevent the wavy web due to flange cooling, a long cooling time is necessary and the productivity becomes unavoidably poor.
The present inventors also found that the cooling of flange according to Japanese Patent Publication No. 1-205028 effectively prevents the wavy web, but sometimes causes a quench hardening of the flange, i.e., the flange surface hardness becomes too high. It is known that boring and other working is generally difficult when an H-beam steel has a too high surface hardness, and particularly, the quench-hardened surface has an extremely high yield point (YP) and tensile strength (TS) and a poor elongation, and thus a desired mechanical property cannot be obtained. The inventors also found that the increase in the surface hardness is most remarkable when producing thin-webbed H-beam steels by using a forcible flange cooling.